In the course of extracting valuable minerals from a mined ore, the ore goes through a number of different processing stages. In the case of some nickel-containing ores, the preferred processing route involves high pressure acid leaching (HPAL) in autoclaves.
The ore is ground to provide a particle size that is suitable for processing and is then formed into a slurry by the addition of recycled process water. The slurry is supplied to an autoclave where sulfuric acid is added. The conditions in the autoclave are controlled depending on the mineralogy of the ore feed to maximize nickel leaching. However, processing conditions in the autoclave generally involve an elevated pressure in the range of 30 to 52 atm, temperatures in the range of 120° C. to 270° C. and acid addition of 200 to 500 kg/t of ore. Agitators are immersed in the hot acidic slurry to achieve suspension of solids.
In order to withstand these conditions, autoclaves are lined with titanium and the agitators are manufactured from titanium alloys but they are subject to considerable abrasion from contact with the slurry. Accordingly, agitators are subject to very abrasive and corrosive conditions and are typically manufactured with a wear resistant coating to improve the operational lifespan of the agitator.
The HPAL process operations are continuous. However, due to wear of agitators and other parts in an autoclave, periodic shutdowns are required to replace worn parts. Typically, this involves shutting down the autoclave for a period of about 3 weeks, including bringing the acid down in temperature and pressure, de-scaling, routine corrosion and wear monitoring, changing over agitators and re-commencing operations. Autoclaves are typically shut down every 9 months so that, amongst other factors, the wear of the agitators can be assessed. If the agitator blade has worn to an extent that agitator efficiency is adversely impacted, the agitator is replaced. If not, the agitator is placed back in service and wear is assessed again in a further 9 months.
Historically, agitators were not coated with a wear resistant coating. They were instead constructed of Grade 5 or Grade 12 titanium.
Wear resistant coatings of titanium dioxide (TiO2) were adopted subsequently to improve the operational lifespan of the agitators. The titanium dioxide coating is applied by thermal spraying of TiO2 particles directly onto an agitator. An example of a microstructure of a TiO2 coating is shown in FIG. 1. The coating provides good wear resistance and it can be applied on-site at the autoclave. However, achieving a good coating requires a high level of preparation work to the agitator surface to ensure that it is free of contaminants. Even then, the TiO2 coating forms a generally poor mechanical bond with the surface. Coating depth is limited to 0.5 mm because it is not possible to build up thicker layers of the coating due to inherent residual stresses within the coating. Due to properties of the TiO2 coating, the coating must be totally removed from the agitator before a fresh coating is applied.
An alternative wear resistant, but not galling resistant, surface for agitators is reaction welded titanium nitride (typically a mixture of titanium and titanium/nitrogen intermetallics). An example of a microstructure of a titanium nitride hard-facing surface is shown in FIG. 2. This hard-facing is formed by producing a molten titanium weld pool on the surface of the agitator substrate and supplying a mixture of nitrogen and argon gas to the weld pool to cause a chemical reaction. As more nitrogen reacts with the titanium, the predominant phases produced change to higher nitrogen containing phases causing the coating to become brittle and porosity levels to increase. Due to the fact that this product is produced by an exothermic chemical reaction, and is limited by kinetic factors, the product is typically heterogeneous. The hardness of this product is not uniform since hardness is related to the diffusion of nitrogen through the molten titanium, which occurs at slower rates farther from the surface.
As shown in FIG. 2, the microstructure is a mixture of various titanium nitride intermetallics and a solid solution containing both titanium and dissolved nitrogen. The titanium nitride intermetallics are hard and provide the reacted surface with good wear resistant properties but poor galling resistant properties. With titanium nitride hard-facing, the reaction depth is generally around 1.5 mm. Additionally, the resultant reacted surface is metallurgically bonded to the substrate. While such bonding is beneficial for ensuring that the hard-facing remains on the agitator throughout the service life, the coating process involves consuming part of the agitator. This is problematic because it can change the tolerances of a product being coated and this can be critical to agitator efficiency. Furthermore, the hardness of the coating is off-set by an increase in brittleness that can lead to micro and macro cracking. Due to dilution of nitrogen into the titanium substrate to depths well below the visual reaction zone, titanium nitrided components are not typically re-nitrided because of the resultant reduction in mechanical properties of the base material.
There is a need for an improved wear resistant surface that is suitable for abrasive and corrosive conditions. It is advantageous for the surface to be able to be reapplied easily without damage to the component.